A general interfacial-energetics-tuning strategy for enhanced artificial photosynthesis

The demands for cost-effective solar fuels have triggered extensive research in artificial photosynthesis, yet the efforts in designing high-performance particulate photocatalysts are largely impeded by inefficient charge separation. Because charge separation in a particulate photocatalyst is driven by asymmetric interfacial energetics between its reduction and oxidation sites, enhancing this process demands nanoscale tuning of interfacial energetics on the prerequisite of not impairing the kinetics and selectivity for surface reactions. In this study, we realize this target with a general strategy involving the application of a core/shell type cocatalyst that is demonstrated on various photocatalytic systems. The promising H2O2 generation efficiency validate our perspective on tuning interfacial energetics for enhanced charge separation and photosynthesis performance. Particularly, this strategy is highlighted on a BiVO4 system for overall H2O2 photosynthesis with a solar-to-H2O2 conversion of 0.73%.

. Co 2p XPS spectra of CoOx/BiVO4. The Co 2p3/2 peak can be deconvoluted to a Co 2+ peak at 781.6 eV and a Co 3+ peak at 780.6 eV. The area of Co 2p3/2 peak is twice of that of Co 2p1/2 peak and the binding energies of Co 3+ and Co 2+ are 780.6 and 781.6 eV, respectively. The other two peaks are the Co satellite peaks.

Section S1. Calculating the photogenerated charge-carrier distribution
The photon flux () from the lamp is shown in Fig. S17a. The reactor except for the window was wrapped by aluminum foils to preserve the light from escaping from the reactor. Therefore, all the incident light with wavelength < 520 nm (the light absorption edge wavelength of BiVO4) is supposed to be absorbed by the BiVO4 particles. As the particles were suspended in solution with stirring, individual particle was supposed to get irradiated homogenously from all direction. The window of the reactor for light illumination has an area of 7 cm 2 . Therefore, the total number of incident photons into the reactor is 7 cm 2 × (). Based on Brunner−Emmet−Teller (BET) analysis, BiVO4 particles exhibit a surface area of 1.5 m 2 /g. During photocatalytic H2O2 generation, the suspension contained 0.05 g of particles, which gives a total surface area of 750 cm 2 . Therefore, the photon flux on the surface of a particle, 0(), is [7 cm 2 × ()]/750 cm 2 , as shown in Fig. S17b. The 0() in the range of  nm which can be absorbed by BiVO4 is integrated and gives a value of 1.7×10 −3 mol s −1 cm −2 . Based on the morphology of the BiVO4 particle used in our simulation (Fig. S16), the surface area of each particle is estimated to be ~ 6.3 ×10 −8 cm 2 . Therefore, the total photon flux absorbed by each particle ( × ) is 6.5×10 7 [#/s]. By dividing this value with the total volume of each particle,   The photocarrier generation × is split into 6 directions as shown in Fig. S18. Given that the light irradiation is uniformly distributed on the particle surface, the generation × for each facet is set to be proportional to their surface areas. As a result, generation × for top/bottom and side facets are 1/4 × × and 1/8 × × , respectively. Here, we calculate the generation rate ( ) dependent on the penetration depth using the z direction as an example (Equations S1-S3).
Note that ( ) here is the summation of wavelength < 520 nm and only depends on the penetration depth. In addition, ( ) only varies along the z axis and is constant along x and y direction.

Section S2. Converting photocatalytic H2O2 generation rates to photocurrent densities
Because H2O2 may decompose as its concentration accumulated, the photocatalytic H2O2 generation rates in the first 5 min of CoOx/BiVO4/Pd and CoOx/BiVO4/(Ag/Pd) was applied for simulation. The H2O2 generation rates as shown in Fig. 2a were converted to photocurrent densities at the reduction and oxidation facets for the convenience of discussion.
The H2O2 generation rate of CoOx/BiVO4/(Ag/Pd) is 3600 M h −1 in 50 mL suspension, corresponding to a photocurrent of 9.63 mA. The suspension contains 0.05 g particles with a total surface area of 750 cm 2 . Based on the SEM image of BiVO4 particles (Fig. S16a), the sum of the areas of top and bottom facets is close to that of all side facets. To this end, the effective area for oxidation or reduction is ~ 375 cm 2 . The photocurrent density for surface reactions is 9.63 mA/375 cm 2 , i.e., 0.0257 mA cm −2 .
The H2O2 generation rate of CoOx/BiVO4/Pd is 1140 h −1 . By the same approach, its photocurrent density for surface reactions is calculated to be 0.0081 mA cm −2 .

Section S3. Simulation model
Following a previous study, 22 we regard a cocatalyst-loaded BiVO4 particle as the combination of a